Illumination systems include a light source and illumination optics for transferring light from the light source to a desired destination. Light from the light source is projected to a target, and the light reflected back is collected to form an image. The light source in an illumination system may be, for example, a light emitting diode (LED) or a laser. Laser light is inherently narrow band and gives rise to the perception of fully-saturated colors and unfortunately, narrow band light incident on random surfaces introduces an unacceptable image artifact known as “speckle.”
Speckle (or speckle noise) may exist when one or more coherent light sources, such as laser light sources, are used in an illumination system. Speckle is typically randomized interference and a characteristic of coherent imaging.
The visual effects of speckle detract from the aesthetic quality of an image and also result in a reduction of image resolution. A variety of “de-speckling” techniques known in the art can be used to reduce this artifact to “acceptable levels,” but only at the expense of system efficiency, which negatively impacts cost, reliability, package size, and power consumption.
Semiconductor lasers can be used to emit light in the near infrared wavelength range for illumination applications such as night-vision, covert surveillance, or industrial illumination. One particular structure of a semiconductor laser is a vertical-cavity surface emitting laser (VCSEL), where light emits perpendicular to the surface of the wafer plane. This vertical structure of the VCSEL lends itself easily towards building 2D arrays with high output power and low beam divergence, and the light from each VCSEL emitter element in the array can be easily collimated and used to build an efficient laser illuminator.
In the prior art, each VCSEL in an array tends to have very close to the same wavelength. This is due to the close proximity of the emitter elements within the array and the uniform wafer characteristics of the epitaxially grown laser structure. Therefore the individual VCSELs in the 2D array form speckle patterns that are nearly identical. These patterns combine to form high contrast speckle. High contrast speckle results in high levels of noise in the image, and limits the ability of an imaging system to resolve fine spatial detail. At worst, without some form of correction, speckle can make coherent illumination unsuitable for display purposes.
When operating near the noise floor of the system, such speckle noise can have a pronounced impact. Current techniques used to break speckle noise, such as utilizing vibrating diffusers and fiber stretchers, can actually introduce additional artifacts that reduce the image quality.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein. An overview of embodiments of the invention is provided below, followed by a more detailed description with reference to the drawings.